EE292: Fundamentals of ECE

Transcription

1 EE292: Fundamentals of ECE Fall 2012 TTh 10:00-11:15 SEB 1242 Lecture

2 2 Outline Review More Diodes Lab Kits

3 3 Diode Voltage/Current Characteristics Forward Bias ( On ) Positive voltage v D supports large currents Modeled as a battery (0.7 V for offset model) Reverse Bias ( Off ) Negative voltage no current Modeled as open circuit Reverse-Breakdown Large negative voltage supports large negative currents Similar operation as for forward bias

4 4 Ideal Diode Model Two state model On State Forward operation Diode is a perfect conductor short circuit Off State Reverse biased No current through diode open circuit Useful for quick and dirty understanding of a complicated circuit Will improve this model to make it more realistic (offset model)

5 5 Circuit Analysis with Diodes Assume state {on, off} for each ideal diode and check if the initial guess was correct i d > 0 positive for on diode v d < 0 negative for off diode These imply a correct guess Otherwise adjust guess and try again Exhaustive search is daunting 2 n different combinations for n diodes Will require experience to make correct guess

6 6 Ideal Diode Example Use the ideal-diode model to analyze the circuit. Start by assuming D 1 is off and D 2 is on.

7 7 Ideal Diode Example D 1 is on short circuit D 2 is off open circuit C Using voltage divider v C = = 6 V v D2 = 3 v c = 3 6 = 3 V Reverse biased off correct operation D 1 current through series resistance i D1 = 10 = 10 = 1 ma > k 10k Current flow forward bias on correct operation

8 8 Offset Diode Model (Simple piecewise-linear diode equivalent circuit in book) Two state model On State Forward operation Diode has a fixed voltage across terminals v f = v on = 0.7 V Off State Reverse biased No current through diode short circuit More realistic than the ideal model Circuit analysis works in the same way as for ideal case Replace on diode with 0.7 V battery

9 9 Rectifier Circuits Convert AC power into DC power These are the basis for power supplies and battery chargers E.g. turning the 60 Hz AC wall power into a 9 V DC voltage for use in a radio

10 10 Half Wave Rectifier Circuit AC source only supplies current to load when the voltage is positive The ideal diode has matches the positive halves of the sine wave Actual rectifiers have a small voltage loss due to the on voltage of real diodes

11 11 Half Wave Rectifier as Battery Charger Current only flows when V in is greater than V B Diode is forward biased ( on ) R is used to limit current into the battery and to avoid destroying the diode

12 12 Rectifier with Smoothing Capacitor Capacitor gets charged by AC source Reverse biased diode does not allow any current from the source Capacitor supplies energy capacitor discharges energy Discharge causes ripple between half wave peaks

13 13 Zener Diode Diode intended to be operated in breakdown Constant voltage at breakdown Three state diode 1. On 0.7 V forward bias 2. Off reverse bias 3. Breakdown v BD reverse breakdown voltage v BD v on = 0.7 V

14 14 Voltage-Regulator Circuits Regulator produces a constant output voltage from a variable DC source E.g. a V battery (voltage lowers as it discharges) and constant 5 V needed for electronic circuits

15 15 Zener Diode Regulator Circuit Select Zener v BD = v o for the desired output voltage Since the diode is in reverse orientation i D cannot be positive For V ss > v o Zener diode is reverse bias Operating in breakdown v D = v BD = v o Remember Zener diodes are designed to operate in breakdown

16 16 Clipper Circuit A D 1 D 3 D 2 D 4 If first D 1, D 2 branch conducting A is higher voltage than B D 1 on 0.6 V drop across it D 2 (reverse biased) operating in breakdown v BD = 5.4 V drop across it v AB = = 6 V If the second D 3, D 4 branch conducting B is higher voltage than A D 4 on 0.6 V drop across it D 3 (reverse biased) operating in breakdown v BD = 8.4 V drop across it v BA = = 9 V, v AB = 9 V B

17 17 Clipper Circuit D 1 D 2 D 3 D 4 v o = v in v in between [6, -9] volts, both paths are not conducting D 1 and D 4 are off